1. Field of the Invention
The present invention relates to the improvements of a turbo-charger control system for a turbocharged internal combustion engine equipped with an exhaust-gas recirculation control system, specifically to techniques for cross-correlation between exhaust-gas recirculation (EGR) control and turbocharger control during EGR addition particularly on diesel engines equipped with a variable displacement turbocharger and an exhaust-gas recirculation system.
2. Description of the Prior Art
A more earlier-model-internal combustion engine is often equipped with a variable displacement (often called as a "variable nozzle") turbocharger in which an inlet opening area (or an opening size) between respective two adjacent vanes or blades of a turbine wheel is variably controlled depending on engine operating conditions. One such variable nozzle control device for a variable-nozzle turbocharger has been disclosed in Japanese Patent Provisional Publication No. 58-176417. On the other hand, almost all of automotive internal combustion engines have exhaust-gas recirculation systems, which are used to reduce nitrogen oxide (NO.sub.x) emissions from exhaust gases of the engine by way of the fall of combustion temperature, caused by recycling of some of the inert exhaust gas back through an intake manifold. One such EGR system has been disclosed in Japanese Patent Provisional Publication No. 60-230555. On one hand, EGR is useful to decrease the formation of NO.sub.x. On the other hand, undesiredly excessive EGR deteriorates combustion, thus dropping engine power output, and also reducing driveability of the vehicle. In recent years, it is desirable to achieve high-precision EGR control in due consideration of various factors, namely engine temperature (during cold engine start, during warming-up period, or in the engine warmed-up state), the reduced effective opening of an EGR control valve occurring owing to lubricating oil (engine oil) or a product of combustion (carbon deposits) adhered to the EGR control valve, changes in an intake-air flow rate occurring due to changes in air density, arising from changes in environment from low-land driving to high-land driving under constant engine speed and load, and the delay in boost pressure (often called "turbo-lag") on turbo-charged engines in a transient state, such as in a transition from normal-straight ahead driving to heavy vehicle acceleration.